The present invention relates to offshore drilling and well activities preformed from a floating drilling or workover rig or vessel. Today, when an offshore sub-sea well is intervened (work performed inside the production tubing below a sub-sea x-mas tree) from a floating vessel, a high pressure workover riser system is used. Such work-over riser systems have been designed with a subsea shut off valve lower riser package and/or a blow out preventer configuration close to the seabed and includes a riser disconnect package (RDP), to allow for a riser disconnect closer to the seabed when situations call for it. On the surface, the high pressure riser is terminated in a surface test tree (series of valves) above the rigfloor. To allow for riser tension, the drilling rig's main blocks for lowering and hoisting drillpipe is used to pull tension on the workover riser. Above the surface test tree, the pressure control equipment (surface BOP) for the well operations is installed, for lubricating into the well all of the work-over tools used in the high pressure operation.
If the work-over system is being used inside a 21″ drilling riser, the lower shutoff valves in the workover riser system close to seabed, are controlled independent of the drilling BOP on the outside and carry independent equipment for service of the well. To run all of this equipment inside the drilling riser is very time consuming, in that the rig crew first has to run the 21″ marine drilling riser and the 18¾ drilling BOP and suspend this system in the drilling rig's riser tension system underneath the rig floor. Then the rig crew has to run the workover riser system inside the marine drilling riser all the way to seabed and connect this riser to the outer drilling subsea BOP in the lower end and suspend this riser system in the rig's main drilling hook by help of an elevator or lifting frame in the upper end. In doing so, the main travelling blocks/hook is occupied and will prevent the rig from being able to run jointed pipe into the workover riser.
If the high pressure riser is run as a stand alone system in open waters, the subsea blowout preventer (BOP) and the riser disconnect package (RDP) is installed on top of the subsea x-mas tree. This riser system is to date not intended for use with jointed drillpipe but intended for extending the production tubing up to the drilling rig's work deck or rigfloor, so that wire line and coil tubing can be run into the well. This riser system is then hung off in the rig's drilling riser tensioning system and/or in the drilling hook with the help of an elevator or lifting frame. The surface BOP's for the workover riser system is then installed above the rig floor and above the elevator to the rigs main hoisting system. This will also prevent the rig from being able to run jointed drillpipe into the well, since the equipment for running jointed pipe is occupied holding tension in the riser system. Hence with prior art, it is not possible to change from running wire line or coiled tubing equipment into the well, into the process of running jointed drillpipe into the well or vice versa, without having to change out the whole riser system or disconnecting the riser from the production sub-sea x-mas tree.
The operating limits upon intervention are as follows: 4 meters of rig heave before disconnect, and riding belt operations are suspended at 1.5 meters of heave at a maximum wind speed of 40 knots of wind. These conditions are quite tight, in particular in harsh climates such as the North Sea. When this is compared operating parameters of drilling being: The drilling operations are stopped if weather conditions are above the following parameters: 5 meters rig heave and increasing, wind above 64 knots. Weather conditions for disconnecting drilling riser: 6-10 meter heave and increasing, Problems with station keeping/High anchor tension, maximum flexjoint angle is 8° and increasing. As is evident, there is a large difference in the operating windows between these parameter sets, thus it would a significant improvement to be able to increase the window of operations for intervention.
Conventional Systems
When completing a well with a conventional vertical X-mas tree system, a dual bore riser is used. The vertical X-mas tree has two bores, production- and annulus bore which contain valves, normally gate valves, in both bores. As a minimum both bores have 1 master valve and 1 swab valve in addition to wing valves and X-over valves etc. none of which form part of the vertical bore. Extended from these two bores the dual bore riser runs all the way back to the rig. The riser is then terminated in the surface test tree or similar which carries an interface which is suspended in the blocks. Above the surface test tree a set of wire line or coiled tubing BOPs are located. Normally only one bore holds the BOPs as the distance between the bores are narrow and there is no room to hold BOPs for both the annulus and production bore. Normally the annulus bore is 2″ nominal and the production bore has an ID of 4″ to 6⅝″.
When a wire line or coiled tubing run is to be performed the BOPs are located on the production or annulus bore and the tool string is inserted by penetrating the BOPs and into the bore. Normally a lubricator is used at the top where the tool string is entered. After having pressure tested the bore and lubricator, the run is performed. When the run is completed in one bore the operation is repeated in the opposite sequence to remove the tool string.
When completing a well or performing Plug and abandonment of a well the need to run plugs in the tubing hanger for isolation and sealing off the well is required. This is then done by installing or removal of a plug in one bore before moving all BOPs etc to the opposite bore for conducting the same operation in that bore. This is a time-consuming operation with personnel subjected to rig heave and weather conditions. Much of the work must be performed in riding belts, operations which in the North sea are limited by the 1.5 meter rig heave and 40 knots wind limitation.